Optical communications networks are becoming increasingly popular for data transmission due to their high bandwidth capacity. Typically, a bit-stream is encoded (e.g., using On-Off-Keying—OOK) to generate sequential symbols that are conveyed through a communications channel by a respective optical channel signal. In most cases, the optical channel signal is generated by a narrow-band optical source (e.g., a narrow-band laser) tuned to a desired channel wavelength. At a receiving end of the communications channel, an optical receiver detects and decodes the symbols of the optical channel signal to recover the original bit-stream. Typically, the receiver is composed of an optical detector followed by electrical signal processing circuitry. The optical detector converts the incoming optical channel signal into a corresponding electrical channel signal. The electrical signal processing circuitry (e.g., Analog-to-Digital (A/D) converter, digital filter, equalizer, Forward Error Correction circuits, etc.) decode the symbols within the electrical channel signal to recover the bit-stream.
In Wavelength-Division Multiplexed (WDM) and Dense Wavelength-Division Multiplexed (DWDM) optical systems, multiple optical channel signals, each of which has a respective different channel wavelength, are multiplexed into a broadband optical signal which is launched through an optical fiber. In order to recover any given bit-stream, the corresponding optical channel signal must be demultiplexed from the broadband optical signal and directed to a receiver for detection and data recovery.
Conventional optical demultiplexers utilize a cascade of wavelength-selective filters, such as Array Waveguide (AWG) or Fiber Bragg Grating (FBG) filters. Each filter operates to extract light within a narrow band centered about a predetermined filter wavelength, which is chosen to correspond to a specific channel wavelength. Filter-based demultiplexers suffer a disadvantage that their design is tightly related to the channel plan of the communications network. Consequently, the channel plan of the system cannot be changed without also replacing every involved optical demultiplexer in the network.
The publication “Polarization Independent Coherent Optical Receiver”, by B. Glance, Journal of Lightwave Technology, Vol. LT-5, No. 2, February 1987, proposes a coherent optical receiver for detecting data traffic encoded within an optical signal. Theoretical considerations relating to the performance and behavior of coherent optical receivers are presented in “Performance of Coherent Optical Receivers”, by John R. Barry and Edward A Lee, Proceedings of the IEEE, Vol. 79, No. 8, August 1990 and “Fiber-Optic Communications Systems”2nd ed. Govind P. Agrawal, John Wiley & Sons, New York, 1997, ISBN 0-471-17540-4, Chapter 6. In general, an optical local oscillator (LO) signal is added to a received optical signal, and the combined lightwave is directed towards a photodetector. The current produced by the photodetector includes an Intermediate Frequency (IF) signal that is centered at an IF equal to the difference between the LO and optical signal frequencies, usually in the microwave (GHz) range, where well established electrical signal processing techniques can be employed to detect and decode the data traffic.
In principle, coherent optical receivers of this type offer the possibility of receiving broadband optical signals without suffering the limitations of conventional filter-based demultiplexing methods. For example, the LO may be tuned to translate any desired optical channel frequency to a predetermined IF to facilitate carrier detection and data recovery, in a manner directly analogous to radio frequency homodyne, heterodyne and super-heterodyne receivers. With this arrangement, changes in the channel plan of the network (in terms of the number of channels and the specific channel wavelengths used) may be accommodated “on the fly” by changing the LO signal wavelength, rather than the receiver equipment itself.
Another expected benefit of coherent receivers is based on their extremely narrow-band data detection performance. In particular, electrical signal filtering of the IF signal typically provides strong attenuation of signal components lying outside of a very narrow frequency band about the predetermined IF, which should enable the receiver to discriminate between closely spaced wavelength channels of a received broadband optical signal.
However, coherent optical receivers suffer a limitation in that their narrow-band performance renders them highly sensitive to carrier offset and phase noise. In fact, optimal data recovery is obtained only when the channel frequency (in the IF signal) exactly corresponds with the predetermined IF. As the channel frequency shifts away from this predetermined value (i.e., as the carrier offset increases), data recovery performance degrades rapidly. Phase noise in either the LO or received optical signals appears as noise in the IF signal, and degrades receiver performance. In order to avoid this problem, and thereby enable satisfactory data recovery, very low noise laser sources (for both the transmitter and the receiver local oscillator) and microwave phase-locked loops are required. This requirement dramatically increases the cost of both transmitters and receivers. As a result, coherent optical receivers are not commonly utilized in modern optical communications networks.
Accordingly, a cost-effective frequency-agile optical transceiver remains highly desirable.